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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

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GYPSUM

161

In less common use is gypsum for plastering or for slow-setting; this,
having been heated at a very high temperature, has lost almost aU its water.
It may contain also calcium hydroxide (instead of carbonate) and calcium
sulphide (from the reduction of the sulphate).

Complete examination of gypsum includes chemical analysis and various
physical and mechanical tests ; in practice, these are mostly reduced to
the determination of the water and the setting test.

1. Chemical Analysis.—The water is determined by heating a weighed
quantity of the sample at 160-180° to constant weight.

In general the impurities may be estimated together with sufficient
accuracy by exhausting a given weight of the substance with hot water
until all the calcium sulphate is dissolved and weighing the residue after
drying.

When complete analysis is required, exactly 1-2 grams are dissolved
in boiling, concentrated hydrochloric acid, the liquid being then diluted
with boiling water, heated for a few minutes longer and filtered, and the
insoluble residue ignited and weighed. In an aliquot part of the filtrate
the sulphuric acid is determined by precipitation with barium chloride,
and in another the alumina and ferric oxide, lime and magnesia are succes-
sively precipitated, as with limestone. If required, the carbon dioxide
may also be determined.

2. Technical Tests.—These consist especially in determinations of
the specific gravity, apparent density, fineness of grinding, amount of water
required for mixing, time of setting and strength; they are made as with
cements.

Properly burnt gypsum contains about 5-7-5% (on the average, 6-5%) of
water and has the specific gravity 2-5-2-7 (usually about 2-68), whereas slow-
setting and almost anhydrous gypsum has the specific gravity 2-7-2-95 (usually
2-93). The proportion of calcium sulphate varies from 80 to 95%. Unless
present in large quantities, the impurities have no injurious influence on the
hardening. Ordinary gypsum sets in a few minutes, whilst the slow-setting form
takes several hours.

A.c.

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